Punters heading to Fieldays this week are being urged to check that their measles immunisations are up-to-date in the wake of an outbreak which has put one high school into lockdown.

Waikato District Health Board's medical officer of health Dr Anita Bell said any time when large crowds gather unimmunised people risk being exposed to the virus.

Bell said she expected to see new cases from other areas given the saturation Fraser High School has with the community. The high school has been quarantined while the DHB's Population Health service investigates almost 45 cases.

"It's about ensuring people check their immunisation status before attending these events. It is best to check with their GP and never assume they have it," Bell said.

Seniors at Fraser High have been left scrambling to reschedule their hair and make-up appointments after administrators temporarily cancelled the July 4 school ball.

A possible 12 new cases of the virus were reported yesterday, all linked with the school in Hamilton's west.

"We are asking anyone who has not had two MMR immunisations be kept at home or away from the school," she said.

Bell urged parents with unimmunised children to think about the risk they were posing to other families.

"They have the potential to be infectious to others, and this is the best way to minimise that risk."

The airborne virus can be contained only by limiting the number of unimmunised people exposed to it, Bell said.

An email from the Fraser High School's principal Virginia Crawford sent to parents stated all extra-curricular activities would be cancelled or postponed for the remainder of the term.

"The exception is if all students are able to produce a medical certificate or medical confirmation that they have been fully immunised," the email said.

All sporting, cultural and academic events have been cancelled for the remainder of term 2 and visitors have been asked to bring proof of their two MMR vaccines.

Amy Jesson's son goes to Fraser High School and said she was relieved her children were immunised.

"I rang up our doctor to make sure my kids were definitely immunised, and let friends with young kids know what was happening," she said.

Jesson said parents at the school had been kept well informed since the outbreak first happened last month. But the resurgence in measles outbreaks across the North Island has reignited the contentious debate surrounding measles-mumps-rubella immunisations.

The Ministry of Health website said despite the seriousness of measles and the proven effectiveness of the MMR vaccine, many parents worldwide became concerned about giving this vaccine to their child because of misleading claims in the 1990s that it [the vaccine] could be linked to autism and inflammatory bowel disease.